1. Field
The present description relates to measuring outgas emissions in fabrication chambers and in particular to sampling outgas emissions during a process and analyzing the outgas samples offline.
2. Background
The manufacture of integrated circuit devices involves the sequential processing of a semiconductor wafer, usually silicon, gallium-arsenide, lithium-niobate or ceramic, to add or remove various layers of material, such as oxides, metals and silicon. The layers are formed by etching, washing, implanting, and depositing.
Photolithography is a process of patterning each layer on the wafer. As many as 35 masks may be required to pattern all of the layers on the wafer. A mask is fabricated by depositing an opaque material, such as chrome, on a transparent or reflective substrate, such as glass or quartz, and then etching the chrome to form features that are many times larger than the desired size on the wafer.
The wafer is covered with photoresist, a material that is sensitive to light. An exposure tool, such as a stepper, shines light through the mask onto the wafer. The optical system reduces the features of the mask so as to print a latent image with the correct dimensions in the photoresist on the wafer. After being exposed, the photoresist is developed to create openings corresponding to the location and intensity of light exposure. Thus, the photoresist serves as a stencil for transferring the pattern, originally in the mask, into the wafer by etching or ion implanting the underlying layers through the openings.
In order to reduce the minimum dimensions of the features on the wafer, the wavelength of the light is reduced and the optical reduction from mask to wafer is increased. These changes require greater accuracy in the optical system that transfers the mask pattern onto the wafer. Contaminants that find their way onto the optical system or onto many other components of the fabrication chamber can reduce the accuracy of the optical system and create errors in the finished product. Over time, the optical system and other components may be destroyed by the contaminants that have been released into a processing chamber by the photoresist, developers and other materials used in the fabrication process.
One source of contaminants is outgassing from the materials used to make the semiconductor. Many materials produce chemicals that are released as gasses into their surrounding environment. In normal room conditions and for some materials, this outgassing may be detectable as odors. The amount of gas produced depends on the environment. Different amounts of outgassing may occur with different light wavelengths, with different light intensities and with different temperatures. For many materials the amount of gas is increased when the material is heated, illuminated and placed in a vacuum. These conditions are common in semiconductor manufacturing and especially during photolithography.
While designers try to select materials with low levels of outgassing, it is difficult to obtain accurate information. Since the amount of outgassing varies in different environments, measurements that are made using test equipment may not reflect the conditions inside an actual fabrication chamber.